Allow is-selected class on <td> and <th> tags (#1380)

* Allow is-active class on <td> and <th> tags

* Fix incorrect selectors

sass/elements/table.sass

@@ -41,6 +41,12 @@ $table-striped-row-even-hover-background-color: $white-ter !default
     &.is-narrow
       white-space: nowrap
       width: 1%
+    &.is-selected
+      background-color: $table-row-active-background-color
+      color: $table-row-active-color
+      a,
+      strong
+        color: currentColor
   th
     color: $table-cell-heading-color
     text-align: left